This invention relates to the synthesis of mercapto-functional organosilicon compounds xe2x89xa1Sixe2x80x94(CH2)nSH, more particularly mercaptoalkylalkoxysilanes such as mercaptopropyltrialkoxy silane (MPTAS), using phase transfer catalysts. The process is capable of producing high purity mercaptopropyltriethoxysilane (MPTES), for example. Mercaptoalkylalkoxysilanes made by the process generally have a formula corresponding to Zxe2x80x94Alkxe2x80x94SH in which Z is one of a group consisting of xe2x80x94SiR12R2, xe2x80x94SiR1R22, and xe2x80x94SiR23; in which R1 is an alkyl group with 1-12 carbon atoms, a cyclohexyl group, or a phenyl group; R2 is an alkoxy group containing 1-12 carbon atoms; and Alk represents a divalent hydrocarbon radical having 1-18 carbon atoms and containing no unsaturation.
Sulfur containing organosilicon compounds are known to be useful as reactive coupling agents between rubber and silica fillers, for improving the properties of cured rubber. They are also known to be useful as adhesion promoters, for adhering rubber compositions to substrates such as glass and metal. However, many sulfur containing organosilicon compounds are difficult to make in good yield, because undesirable byproducts are produced from various side reactions occurring when traditional methods are employed.
For example, U.S. Pat. No. 3,590,065 (Jun. 29, 1971) relates to a reaction between a haloalkylalkoxysilane and a thio-urea in the presence of ammonia. However, the necessity of handling bulky by-products such as guanidine hydrochloride is the major disadvantage associated with this particular method.
U.S. Pat. No. 3,849,471 (Nov. 19, 1974) is directed to another method involving reactions between haloalkylalkoxysilanes and hydrogen sulfide gas in the presence of amines. However, this reaction is carried out under a high pressure, and also has the disadvantage of producing a fluffy by-product salt which is difficult to filter from the end product.
U.S. Pat. No. 3,890,213 (Jun. 17, 1975) relates to yet another method involving reactions between hydrogen sulfide and alkenylalkoxysilanes. However, the major disadvantage associated with this method is that the mercaptosilane product itself can become associated with the alkenylalkoxysilane to form copious amounts of sulfide side products.
In British Patent 1,102,251 (Feb. 7, 1968), a method is described involving the reaction of sodium methoxide and hydrogen sulfide to produce sodium hydrosulfide, which is then further reacted with an haloalkylalkoxysilane. The disadvantage associated with this particular method however, is that the reaction of sodium methoxide with H2S produces sodium sulfide as a by-product, which in turn leads to large amounts of polysulfide silanes in the end product.
U.S. Pat. No. 5,583,245 ((Dec. 10, 1996) describes a process for making compounds generally corresponding to the formula Zxe2x80x94Alkxe2x80x94Snxe2x80x94Alkxe2x80x94Z, in which Z and Alk are the same as defined above, and in which n is 2-8. According to the process in the ""245 patent, a compound of the formula Zxe2x80x94Alkxe2x80x94X where X is chlorine or bromine, is reacted with an ammonium hydrosulfide or an alkali metal hydrosulfide, and sulfur, using a phase transfer catalyst, in an aqueous phase.
The ""245 patent teaches that an additional reactant corresponding to Alkxe2x80x94X may be present, where an unsymmetrical compound corresponding to Alkxe2x80x94Sxe2x80x94Alkxe2x80x94Z is desired, in addition to bis type end products. While the ""245 patent does describe a method for preparing MPTES in a 64.9% yield by reacting (i) sodium hydrosulfide flakes and (ii) chloropropyltriethoxysilane (CPTES), in a saturated sodium chloride solution and toluene solvent, in the presence of a phase transfer catalyst, the yield of MPTES was not optimal.
While U.S. Pat. No. 5,840,952 ((Nov. 24, 1998) describes a method of making mercaptopropylalkoxysilanes in good yield, by purging hydrogen sulfide gas in a sodium sulfide solution in methanol, and then reacting it with chloropropyltrimethoxysilane (CPTMS) in an anhydrous system, the disadvantage associated with this process is that sodium sulfide used must first be dehydrated. Another disadvantage of the ""952 patent is that it requires the use of high pressure, i.e., 600 psi/4,140 kilopascal (kPa) hydrogen sulfide gas, to reduce sodium sulfide to sodium hydrosulfide.
In a prior copending application assigned to the same assignee as the present invention, i.e., U.S. patent application Ser. No. 09/895,719, filed Jun. 29, 2001, and entitled xe2x80x9cPreparation of Sulfur Containing Organosilicon Compounds Using a Buffered Phase Transfer Catalysis Processxe2x80x9d, there is described a process based on phase transfer catalysis. However, this process is directed to the production of bis-type sulfido silanes xe2x89xa1Sixe2x80x94Sxe2x80x94Sixe2x89xa1, which generally correspond to the formula (RO)3xe2x88x92mRmSixe2x80x94Alkxe2x80x94Snxe2x80x94Alkxe2x80x94SiRm(OR)3xe2x88x92m, wherein R is a monovalent hydrocarbon group with 1-12 carbon atoms; Alk represents a divalent hydrocarbon group with 1-18 carbon atoms; m is 0-2; and n is 1-8. According to the process described in the copending application, (A) a sulfide compound M2Sn or MHS wherein H is hydrogen, M is ammonium or an alkali metal, and n is as defined above, is reacted with (B) a silane compound corresponding to (RO)3xe2x88x92mRmSixe2x80x94Alkxe2x80x94X, wherein X is Cl, Br or I, m is the same as defined above, and (C) sulfur, in the presence of a phase transfer catalyst, in an aqueous phase containing a buffering agent. However, no method is described in the copending application for making mercapto-functional organosilicon compounds, i.e., compounds containing the group
xe2x80x83xe2x89xa1Sixe2x80x94(CH2)nSH.
The copending application, however, results in sulfidosilanes instead of mercaptosilanes. This is because of the presence of elemental sulfur in the copending application, and the use of different buffering agents in the copending application than the buffering agents (i.e., pH adjusting agents) used in the present application. In addition, pH is a controlling factor in these applications as to what is being prepared. The difference is based on (i) establishment of the equilibrium 
or (ii) the equilibrium 
and (iii) the fact that the disulfide anion is undetectable at a pH of about 9 or less.
Another way to view the difference is that when it is desired to make mercaptosilanes, rather than sulfidosilanes, the alkalinity of the reaction mixture must remain at a pH in the range of 4 to 9. Higher concentrations of alkalinity lead to disulfide anion formation from the mercaptide anion already present, without adding elemental sulfur. When it is desired to make the disulfide, it is necessary for the system to remain high in alkalinity, to inhibit any equilibrium leading to NaHS formation. As these reactants naturally react and form a neutral NaCl, the alkalinity of the brine will lessen over time, and can become low enough so that SH will form.
So the present application differs from the copending application in that (i) different pH adjusting agents are employed, (ii) the pH is different, i.e., a pH of 4 to 9, preferably a pH of 5-8, and more preferably a pH of 5 to less than 7, instead of a pH of 7-14, and (iii) the order of addition of the reactants is not the same. The result is that at the lower pH of 4 to less than 7, any sulfide present is converted to mercaptan in the aqueous phase.
The process of the present invention further differs from processes described above, in that it is capable of providing high yields of mercaptosilanes under mild conditions, without the use of solvents, toxic gas, and strictly anhydrous conditions. In addition, it is more economical, environment friendly, and capable of utilizing relatively inexpensive starting materials. When there is any byproduct present, it is simply an alkali metal salt, which can be easily removed by dissolution during the water and phase separation sequence of the process.
Other advantages provided by the present invention over the prior art include the benefit that the mercaptoalkylalkoxy silane yield is significantly increased when chloroalkylalkoxysilanes are reacted with an aqueous solution of sodium hydrosulfide in the presence of (i) a phase transfer catalyst and (ii) gases which form an acidic solution in water to control the pH of the aqueous phase, at a pressure of 10-200 psi/69-1,380 kPa, preferably 25-100 psi/173-690 kPa. Some particularly suitable gases are hydrogen sulfide, carbon dioxide, and sulfur dioxide. The reaction can be carried out without requiring use of a solvent, and no extra salt is needed to saturate the aqueous phase to prevent hydrolysis of any alkoxy groups present on silicon atoms in the molecule.
Most significantly, however, the order of addition of the reactants plays an important role in the product yield and quality. Thus, it was surprising to discover that the mercaptoalkylalkoxysilane yield can be increased when the NaHS solution is added to a mixture of the haloalkylalkoxysilane and the anhydrous phase transfer catalyst at the reaction temperature. In this regard, anhydrous pH adjusting salts may be required to be added to the organic reaction mixture before the NaHS addition, to control any potential side reactions. In those instances where it is desired to carry out the reaction under pressure conditions, the reactor is pressurized with H2S, CO2, and/or SO2, before addition of the NaHS to the haloalkylalkoxysilane/catalyst mixture. The anhydrous salt of a mineral acid and the phase transfer catalyst can then be mixed with the haloalkylalkoxysilane, before adding the sodium hydrosulfide. This feature will tend to minimize any hydrolysis of alkoxy groups on silicon.
Lastly, the presence of side products of species such as 3,3xe2x80x2bis(triethoxysilylpropyl)monosulfide (TESPM) and 3,3xe2x80x2bis(triethoxysilylpropyl)disulfide (TESPD), are significantly reduced when mineral acids and their salts are used in the process. Thus, the acids and the pH adjusting agents having pH values of less than seven, react with disodium sulfide impurities in the aqueous solution of sodium hydrosulfide, and produce sodium hydrosulfide and hydrogen sulfide gas. The maintenance of a positive pressure of H2S in the existing headspace will prevent the formation of the sulfide species, by shifting the equilibrium between NaHS and sodium sulfide (Na2S), toward NaHS in the aqueous phase.
This invention is directed to a process for making high purity mercaptoalkylalkoxysilanes using a phase transfer catalyst, by forming a mixture of (i) an haloalkylalkoxysilane and (ii) a phase transfer catalyst, prior to the addition of (iii) a sulfide compound such as sodium hydrosulfide. The reaction will occur without requiring that there be present in the haloalkylalkoxysilane phase, a concentrated sodium chloride salt solution with the sodium hydrosulfide, or an organic solvent. The formation of byproducts can also be reduced by the addition of certain pH adjusting agents having a pH of less than seven to the mixture containing the haloalkylalkoxysilane and the phase transfer catalyst. In particular, the process is directed to the reaction of an aqueous solution of sodium hydrosulfide with CPTES in the presence of a phase transfer catalyst, and a pH adjusting agent of a pH of 4 to 9, preferably a pH of 5-8, and more preferably a pH of 5 to less than 7. Thus, it was surprisingly discovered that both the order of addition of the reactants, and the addition of certain pH adjusting agents in the reaction mixture, significantly increased the yield of MPTES by minimizing formation of undesirable end products such as TESPM and TESPD.
As an additional feature, the invention is directed to the use of certain pH adjusting agents which are acidic in their nature in an aqueous solution, such as SO2 (sulfur dioxide), CO2 (carbon dioxide), H2S (hydrogen sulfide), H3PO4 (phosphoric acid), H3BO3 (boric acid), and HCl (hydrochloric acid). These materials improve the yield by minimizing the production of any undesirable byproduct such as TESPM and TESPD.
In a first embodiment of the process for making the mercaptoalkylalkoxysilanes, a pH adjusting agent and a sulfide containing compound are mixed in an aqueous phase to provide a pH of 4-9, a phase transfer catalyst is added to the aqueous phase, a haloalkylalkoxysilane is then added to the aqueous phase to form a reaction mixture containing mercaptoalkylalkoxysilanes and water soluble byproducts, and the desired mercaptoalklyalkoxysilanes are separated from the water soluble byproducts.
In an alternate embodiment, the haloalkylalkoxysilane, the phase transfer catalyst, and an anhydrous pH adjusting agent such as sulfur dioxide, carbon dioxide, hydrogen sulfide, phosphoric acid, boric acid, and hydrochloric acid, and anhydrous salts thereof, are mixed, then an aqueous solution of a sulfide containing compound is added to form a reaction mixture containing mercaptoalkylalkoxysilanes and water soluble byproducts, and desired mercaptoalklyalkoxysilanes are separated from water soluble byproducts.
Following is a list of acronyms used in this application:
CPTESxe2x80x94Chloropropyltriethoxysilane
CPTMSxe2x80x94Chloropropyltrimethoxysilane
MPTASxe2x80x94Mercaptopropyltrialkoxysilane
MPTESxe2x80x94Mercaptopropyltriethoxysilane
TBABxe2x80x94Tetrabutylammonium Bromide
TBACxe2x80x94Tetrabutylammonium Chloride
TESPDxe2x80x943,3xe2x80x2bis(triethoxysilylpropyl)disulfide
TESPMxe2x80x943,3xe2x80x2bis(triethoxysilylpropyl)monosulfide
These and other features of the invention will become apparent from a consideration of the following detailed description.
The synthesis of mercapto-functional alkoxysilanes is carried out via a reaction using a phase transfer catalysis process in an aqueous/organic medium. The use of small amounts of gases such as H2S, CO2, and SO2, as well as the use of the other pH adjusting agents of the invention which are all acidic in nature in the aqueous phase, during the reaction, minimizes the production of undesirable end products such as TESPM and TESPD. Therefore, the yield of a mercaptopropyltrialkoxysilane can be significantly increased to more than 90 percent. As noted, it was also surprisingly discovered that a positive pressure of H2S or CO2 during the reaction of (i) chloropropyltriethoxysilane with (ii) a sodium hydrosulfide solution and (iii) tetrabutylammonium bromide catalyst (TBAB) minimized TESPM and TESPD to less than 1.0 weight percent, compared to about 10 weight percent when no pressure was maintained.
In this regard, it is believed that the H2S or CO2 pressure causes a reduction in the pH of the aqueous phase either by the dissolution of H2S, or there occurs an intermediate reaction of CO2 with H2O to form carbonic acid. In any event, at a reduced pH, the di-sodium sulfide impurities are converted to sodium hydrosulfide, and minimize TESPM formation. Thus, when a sodium hydrosulfide aqueous solution was added to a heated reactor pressurized with H2S gas and containing TBAB and CPTES, the organic phase after filtration included more than 90 weight percent of desired end product MPTES, and less than one weight percent of undesired species TESPM and TESPD.
Accordingly, the first sequence of steps of the process involves mixing a phase transfer catalyst with an haloalkylalkoxysilane. Phase transfer catalysts suitable for use according to the invention are quaternary onium cations. Some representative examples of quaternary onium salts yielding quaternary ammonium cations that can be used as phase transfer catalysts are described in U.S. Pat. No. 5,405,985 (Apr. 11, 1995), among which are tetrabutylammonium bromide (TBAB), tetrabutylammonium chloride (TBAC), tetrabutylphosphonium bromide, tetrabutylphosphonium chloride, tetraphenylarsonium bromide, and tetraphenylarsonium chloride. The ""985 patent is considered as being incorporated herein by reference. The preferred quaternary onium salts according to this invention are TBAB and TBAC, most preferably TBAB. These materials are available commercially from chemical suppliers such as Sigma-Aldrich, Milwaukee, Wis. While the amount of phase transfer catalyst used in the process can vary, it is preferably used in an amount of 0.1-10 weight percent, most preferably 0.5-2 weight percent, based on the amount of haloalkylalkoxysilane being used in the process.
Haloalkylalkoxysilanes for purposes of this invention are those organosilicon compounds having a structure generally corresponding to the formula (RO)3xe2x88x92mRmSixe2x80x94Alkxe2x80x94X, wherein each R is an independently selected hydrocarbon group containing 1-12 carbon atoms, such as methyl, ethyl, propyl, butyl, isobutyl, cyclohexyl, or phenyl. Preferably, R is a methyl or ethyl group. m can be 0, 1, or 2, but preferably m is zero. Alk represents a divalent hydrocarbon group containing 1-18 carbon atoms, preferably 2-4 carbon atoms, such as ethylene, propylene, butylene, or isobutylene. Preferably, Alk is a propylene group. X is one of the halogen atoms, i.e., fluorine, chlorine, bromine, or iodine, preferably chlorine. Some representative examples of haloalkylalkoxysilanes suitable for use in this invention include chloropropyltriethoxysilane, chloropropyltrimethoxysilane, chloroethyltriethoxysilane, chlorobutyltriethoxysilane, chloroisobutylmethyldiethoxy silane, chloroisobutylmethyldimethoxysilane, and chloropropyldimethylethoxysilane. The haloalkylalkoxysilane compound most preferred is chloropropyltriethoxysilane.
The second sequential step of the process is addition of a sulfide compound. The sulfide compound is a composition having a structure corresponding to the formula M2Sn or MHS, or mixtures thereof, wherein M represents an alkali metal or ammonium group, and H is hydrogen. While the alkali metal can be potassium, sodium, rubidium, or cesium, sodium is preferred. Representative of some preferred compositions of the type MHS include compositions such as NaHS, KHS, and NH4HS, with NaHS being most preferred. The NaHS composition can be used in the form of NaHS flakes containing 71.5-74.5 weight percent NaHS, or an NaHS liquor containing 45-60 weight percent NaHS. Such materials are available commercially from PPG Industries, Inc., Pittsburgh, Pa. Optionally, compositions of the type M2Sn can be used, when it is desired to avoid the necessity of dissolving solid or flake forms. Suitable compositions of this type include Na2S, K2S, Cs2S, (NH4)2S, Na2S2, Na2S3, Na2S4, Na2S6, K2S2 K2S3, K2S4, K2S6, and (NH4)2S2. A particularly preferred sulfide composition is a solution containing 25-72 weight percent of NaHS, preferably 45-60 weight percent NaHS, also available from PPG Industries, Inc., Pittsburgh, Pa.
If desired, sulfur (S) can be added as an optional ingredient. A suitable sulfur is elemental sulfur in the form of an 100 mesh refined sulfur powder available from Sigma-Aldrich, Milwaukee, Wis. While the amount of sulfur and sulfide compound can vary, it can be present in a molar ratio corresponding to S/M2Sn or S/MHS of 0 to 2.0, preferably zero.
In the preferred embodiment, the phase transfer catalyst and haloalkylalkoxysilane are combined with a pH adjusting agent which is only slightly acidic in nature or one that is diluted to such an extent as to be only slightly acidic. The pH adjusting agent can be SO2 (sulfur dioxide), CO2 (carbon dioxide), H2S (hydrogen sulfide), H3PO4 (phosphoric acid), H3BO3 (boric acid), or HCl (hydrochloric acid). While the amount of pH adjusting agent added to the aqueous phase can vary, it is generally present in a molar amount of 0.01 to 1.0 mole per mole of M2Sn or MHS being used in the process, preferably 0.01 to 0.3 mole per mole of M2Sn or MHS being used in the process.
The process is carried out in an aqueous/organic phase containing the haloalkylalkoxysilane, phase transfer catalyst, pH adjusting agent, and sulfide compound. While the amount of water used to create the aqueous phase can vary, it is preferably based on the amount of haloalkylalkoxysilane being used in the process. The water can be added directly, or it can be present indirectly, as the water present in small amounts in starting materials. In any case, the total amount of water for purposes of the invention should include all water added directly or indirectly. Accordingly, the total amount of water used to create the aqueous phase is 1-100 weight percent of the amount of haloalkylalkoxysilane being used in the process, preferably 2.5-70 weight percent, most preferably 20-40 weight percent.
Although not being willing to be bound by any particular theory, it is believed that that the addition of only certain pH adjusting agents, i.e., the pH adjusting agents noted above, to the aqueous phase during the process, controls the pH of the reaction medium so as to directly affect product formation and minimizes any potential of undesired side reactions. Thus, the pH is controlled by addition of such pH adjusting agents at rates and concentrations so as to maintain the pH during the reaction in the range of 4 to 9, preferably in the range of 5-8, and more preferably in the range of 5 to less than 7. The haloalkylalkoxysilane compound is added to the aqueous phase at such a rate so as to control the exothermic reaction, and at the same time maintain a temperature in the range of 40-110xc2x0 C. Preferably the reaction temperature is maintained at 60-95xc2x0 C. The progress of the reaction can be monitored by determining the consumption of the haloalkylalkoxysilane. The amount of catalyst being used as well as the reaction temperature will affect the reaction time necessary for its completion.
At the end of the reaction, a product mixture is produced containing an organic phase, an aqueous phase, and possibly some precipitated solid materials that includes salts formed during the reaction. The organic phase contains the mercaptoalkylalkoxysilane, and separation of the mercaptoalkylalkoxysilane from the product mixture can be obtained simply by phase separation of the organic phase from the aqueous phase, or if precipitated salts are formed during the reaction, the salts can be separated first by filtering or decanting prior to the phase separation. Water or a dilute acidic solution can be added to the product mixture prior to separation, as the addition of water or a dilute acidic solution tends to enhance phase separation by dissolving precipitated salts.
The amount of water or dilute acidic solution added during this step can vary from 10-50 weight percent based on the weight of the haloalkylalkoxysilane, preferably 20-40 weight percent, and most preferably 25-35 weight percent. When a dilute acidic solution is used, it can contain HCl, HNO3, or H2SO4, for example, having normal (N) concentrations of 0.000001-5, preferably 0.01-1. The dilute acidic solution can also be prepared by adding a chlorosilane to water, i.e., xe2x89xa1Sixe2x80x94Cl+Hxe2x80x94OHxe2x86x92xe2x89xa1Sixe2x80x94OH+HCl.
Following addition of water or a dilute acidic solution to the product mixture, the mercaptoalkylalkoxysilane can be isolated from the product mixture by phase separating the organic phase from the aqueous phase. The organic phase containing the mercaptoalkylalkoxysilane can also be subjected to a drying step. One example of drying is to treat the organic phase under vacuum to remove any volatile organic materials present, along with the residual water. The drying can be to simply heat the organic phase to a temperature of 20-160xc2x0 C. under a reduced pressure of 5-35 mm Hg (0.67 to 4.65 kPa), preferably 90-120xc2x0 C. at 5-25 mm Hg (0.67 to 3.33 kPa). Alternatively, drying of the organic phase can be obtained using a thin film stripper to remove volatile organic materials as well as residual water in the organic phase.
Yet another drying technique is to contact the organic phase containing the mercaptoalkylalkoxysilane with a desiccant. The desiccant can be any solid material known to be capable of removing trace quantities of water from organic phases. Representative desiccants are typically ionic hygroscopic compositions such as sodium sulfate, magnesium sulfate, as well as the silicate based compositions such as zeolites, silica, and alumina/silicates. Preferred desiccants are sodium sulfate or magnesium sulfate, and sodium sulfate is most preferred.
The dried organic phase can then be subjected to additional steps to further improve its final purity and appearance. Thus, the organic phase containing the mercaptoalkylalkoxysilane can be heated under vacuum to strip low boiling components such as ethanol, water, tributylamine, and dissolved H2S gas, For example, the organic phase, when heated to 30-100xc2x0 C. at 10-200 mm Hg/1.3-27 kPa vacuum, provides a clear product which has a better shelf life. The organic phase, after stripping the low boiling components can also be distilled under high vacuum, i.e., 1-20 mm Hg/0.133-2.7 kPa, to provide highly pure mercaptoalkylalkoxysilanes. As a result, the long-term storage stability of the mercaptoalkylalkoxysilane is enhanced, i.e., the composition does not change with time or result in products containing undesirable solid precipitates.